In methods of forming integrated circuits, it is frequently desired to electrically isolate components of the integrated circuits from one another with an insulative material. For example, conductive layers can be electrically isolated from one another by separating them with an insulative material. Also, devices which extend into a semiconductive substrate can be electrically isolated from one another by insulative materials formed within the substrate and between the components, such as, for example, a trench isolation region.
A suitable insulative material for isolating components of integrated circuits is silicon dioxide, which has a dielectric constant of about 4. In some applications it is desired to lower the dielectric constant of a silicon dioxide containing material to reduce parasitic capacitance. A method of lowering a dielectric constant of a silicon dioxide material is to provide dopant atoms within the material. An example dopant atom is carbon.
A recently developed technique for forming silicon dioxide is a Flowfill.TM. technology, which has been developed by Trikon Technology of Bristol, U.K. In such process, SiH.sub.4 and H.sub.2 O.sub.2 are separately introduced into a chemical vapor deposition (CVD) chamber, such as a parallel plate reaction chamber. The reaction rate between SiH.sub.4 and H.sub.2 O.sub.2 can be moderated by the introduction of nitrogen into the reaction chamber. A wafer is provided s within the chamber, and ideally maintained at a suitably low temperature, such as 0.degree. C., at an exemplary pressure of 1 Torr to achieve formation of a silanol-type structure of the formula Si(OH).sub.x, which is predominately Si(OH).sub.4. The Si(OH).sub.4 condenses onto the wafer surface. Although the reaction occurs in the gas phase, the deposited Si(OH).sub.4 is in the form of a very viscous liquid which flows to fill very small gaps on the wafer surface. In applications where deposition thickness increases, surface tension drives the deposited layer flat, thus forming a planarized layer over the substrate.
The liquid Si(OH).sub.4 is typically converted to a silicon dioxide structure by a two-step process. First, planarization of the liquid film is promoted by increasing the temperature to about 100.degree. C., while maintaining the pressure of about 1 Torr, to result in solidification and formation of a polymer layer. Thereafter, the temperature is raised to approximately 450.degree. C., while maintaining the pressure of about 1 Torr, to form SiO.sub.2. The processing at 450.degree. C. also provides the advantage of driving undesired water from the resultant SiO.sub.2 layer. Unfortunately, SiO.sub.2 formed according to the above-described method is typically less dense than SiO.sub.2 formed by other methods, and will have an unacceptably, and frequently unpredictably, fast etch rate. Also, the low density of the SiO.sub.2 layer can adversely affect a dielectric constant of the layer.
It would be desirable to develop alternative methods of forming SiO.sub.2 which alleviate one or more of the above-described problems.